In physics, what is the improbility level before it is considered "Phyiscaly impossible?"

Question

I know there is a specefic number

Answer 1

It has to be infinitely improbable.

Answer 2

What? No, there is no specific number. We don't go around saying that things are impossible because they are improbable. That, in fact, is a logical contradiction. That is not the way probability works.

That would be like winning the jackpot in the lottery and being turned away at the lottery office because the chances of you winning were 1 in a trillion, and therefore you couldn't have won.

Answer 3

There is no such number.  Impossible is not a degree of improbable.  Having a low-energy between two neutrons result in four neutrons is impossible.  There are physical phenomena like tunneling which appear to be impossible, yet they happen.  It was a paradox until we understood quantum physics.

Answer 4

The Heisenberg Uncertainty principle is a referenced "physically impossible" level.  Basically, it states that the change in position multiplied by the change in momentum must be greater than a constant value.  If it is less, then we have no way to know that the change occurred.

To put in terms we understand, it says that the probability that something has changed must be above a certain value.